This article talks about what a dynamic link library (DLL) is and the different problems that can happen when you use them. It also describes some advanced issues that you should consider when developing your own DLLs.
As well as explaining what a DLL is, this article talks about dynamic linking methods, DLL dependencies, DLL entry points, exporting DLL functions, and tools for fixing DLL problems.
For the Windows operating systems, much of the functionality of the operating system is provided by DLL. When you run a program on one of these Windows OSs, DLLs may provide a lot of the program’s functionality. As an example, some programs may have a lot of different modules. Each module of the program is stored and shared in a DLL.
The use of DLLs helps promote modularization of code, code reuse, efficient memory usage, and reduced disk space. This means that the computer’s operating system and programs load and run faster and use less disk space.
If a program depends on a DLL, it might not be able to run. This is known as dependency. When a program uses a DLL, a dependency is created. If another program overwrites and breaks this dependency, the original program may not successfully run.
With the introduction of the .NET Framework, most dependency problems have been eliminated by using assemblies.
Many programs can use the same DLL library at the same time because it has code and data that many programs can use. For example, in Windows operating systems, the Comdlg32 DLL performs common dialog box related functions. Each program can use the functionality that is contained in this DLL to implement an Open dialog box. It helps promote code reuse and efficient memory usage.
By using a DLL, a program can be modularized into separate components. For example, an accounting program may be sold by module. Each module can be loaded into the main program at run time if that module is installed. Because the modules are separate, the load time of the program is faster. And a module is only loaded when that functionality is requested.
Additionally, updates are easier to apply to each module without affecting other parts of the program. For example, you may have a payroll program, and the tax rates change each year. You can make changes to a DLL and then apply an update without having to build or install the whole program again.
The following list describes some of the files that are implemented as DLLs in Windows operating systems:
The following list describes some of the advantages that are provided when a program uses a DLL:
When a program or a DLL uses a DLL function in another DLL, a dependency is created. The program is no longer self-contained, and the program may experience problems if the dependency is broken. For example, the program may not run if one of the following actions occurs:
These actions are known as DLL conflicts. If backward compatibility is not enforced, the program may not successfully run.
The following is a list of changes that have been made to Windows 2000 and later versions of Windows to help reduce dependency problems:
Several tools are available to help you troubleshoot DLL problems. The following tools are some of these tools.
The Dependency Walker tool can recursively scan for all dependent DLLs that are used by a program. When you open a program in Dependency Walker, Dependency Walker does the following checks:
By using Dependency Walker, you can document all the DLLs that a program uses. It may help prevent and correct DLL problems that may occur in the future. Dependency Walker is located in the following directory when you install Visual Studio 6. 0:
Are you preparing for an interview on Dynamic Link Libraries (DLLs)? Look no further! This guide compiles the most frequently asked DLL interview questions, along with insightful answers, to help you ace your interview and showcase your expertise
What is a DLL?
A DLL or Dynamic Link Library is a collection of code and data that can be shared and reused by multiple programs simultaneously. This modular approach promotes code reusability, reduces memory footprint, and enhances application performance.
Why are DLLs used?
DLLs offer several advantages
- Code Reusability: DLLs eliminate the need to rewrite common code, promoting modularity and efficient development.
- Reduced Memory Footprint: Sharing code through DLLs minimizes memory usage, leading to faster program execution and improved system performance.
- Simplified Updates: Updates to DLLs can be applied without modifying the entire program, simplifying maintenance and deployment.
- Modular Architecture: DLLs enable modular program design, facilitating easier development and maintenance of complex applications.
Types of DLLs:
- System DLLs: These DLLs are essential for Windows operation and are located in the system directory. Modifying or deleting these DLLs can lead to system instability.
- Application DLLs: These DLLs are specific to individual applications and reside in the application’s directory.
- Shared DLLs: These DLLs are shared by multiple applications and are typically located in the Windows system directory or a shared location.
DLL Dependencies
When a program uses a DLL function, a dependency is created. If the dependent DLL is changed, deleted, or updated, the program might have problems. Windows has features like Windows File Protection and Private DLLs to keep dependency problems to a minimum.
DLL Troubleshooting Tools:
Several tools can assist in troubleshooting DLL issues, including:
- Dependency Walker: Scans for missing or invalid DLLs, checks import/export function compatibility, and identifies circular dependencies.
- DLL Universal Problem Solver (DUPS): Audits, compares, documents, and displays DLL information.
- DLL Help database: Locates specific versions of DLLs installed by Microsoft software products.
DLL Development:
Developing your own DLLs requires careful consideration of the following aspects:
- Types of DLLs: Choose between load-time and run-time dynamic linking based on your application’s requirements.
- DLL Entry Point: The entry point function can initialize or destroy data structures as needed.
- Exporting DLL Functions: Use the
__declspec(dllexport)keyword or a module definition file to export DLL functions.
Sample DLL and Application:
Visual C++ offers project types for creating DLLs, such as the Win32 Dynamic-Link Library project type. The provided code examples demonstrate how to create and use a simple DLL in a Win32 application.
DLL vs. .NET Framework Assemblies:
The .NET Framework introduced assemblies, which offer several advantages over DLLs, including:
- Self-Describing: Assemblies contain all necessary information for the CLR to run them, ensuring consistency and compatibility.
- Versioning: Assemblies enforce version information, simplifying version management and preventing conflicts.
- Side-by-Side Deployment: Multiple applications can use different versions of the same assembly simultaneously.
- Self-Containment and Isolation: Assemblies promote application isolation and zero-impact installations.
- Execution Security: Assemblies run under controlled permissions, enhancing security.
- Language Independence: Assemblies can be developed using any supported .NET language.
Understanding DLLs is crucial for developers working with Windows applications. This guide provides a comprehensive overview of DLLs, covering their functionalities, advantages, types, dependencies, troubleshooting tools, development considerations, and comparison with .NET Framework assemblies. By mastering these concepts, you can confidently approach any DLL-related interview questions and showcase your expertise in this essential Windows programming element.
Additional Resources:
- Microsoft Learn: Dynamic Link Library (DLL)
- EQuestionAnswers: DLL Questions and Answers
- Visual Studio: Creating a DLL
Remember:
- Practice answering these questions out loud to improve your fluency and confidence.
- Research common DLL interview questions and prepare tailored responses.
- Highlight your understanding of DLL concepts and demonstrate your problem-solving abilities.
By following these tips and utilizing the provided resources, you’ll be well-equipped to ace your DLL interview and impress your potential employer with your deep understanding of this fundamental programming concept.
The DLL entry point
When you create a DLL, you can optionally specify an entry point function. When processes or threads connect to or disconnect from the DLL, the entry point function is called. You can use the entry point function to set up data structures or delete them, depending on what the DLL needs. If the program runs on more than one thread, you can use thread local storage (TLS) to give each thread its own memory in the entry point function. The following code is an example of the DLL entry point function.
Should the entry point function return a FALSE value, the load-time dynamic linking application will not start. If you are using run-time dynamic linking, only the individual DLL will not load.
The entry point function should only do basic initialization work and shouldn’t call any other functions to load or end the DLL. For instance, you shouldn’t call the LoadLibrary or LoadLibraryEx functions directly or indirectly in the entry point function. Additionally, you should not call the FreeLibrary function when the process is terminating.
For applications that use more than one thread, make sure that all access to the DLL global data is coordinated (thread safe) so that data doesn’t get lost. To do this, use TLS to provide unique data for each thread.
You can either add a function keyword to the exported DLL functions or make a module definition in order to export DLL functions. def) file that lists the exported DLL functions.
Before you can use a function keyword, you need to use the __declspec keyword to declare each function you want to export.
You must use the keyword __declspec(dllimport) to tag each function you want to import in order for the application to use exported DLL functions.
These days, you would separate the export statement and the import statement with a header file that has a define statement and an ifdef statement.
You can also use a module definition file to declare exported DLL functions. You don’t need to add the function keyword to the exported DLL functions when you use a module definition file. In the module definition file, you declare the LIBRARY statement and the EXPORTS statement for the DLL. The following code is an example of a definition file.
DLL Universal Problem Solver
The DLL Universal Problem Solver (DUPS) tool is used to audit, compare, document, and display DLL information. The following list describes the utilities that make up the DUPS tool:
- Dlister. exe This tool lists all of your computer’s DLLs and saves the details to either a text file or a database file.
- Dcomp. exe This program takes two text files with lists of DLLs and makes a third text file with the differences between them.
- Dtxt2DB. exe This program opens the text files that were made with the Dlister. exe utility and the Dcomp. exe utility into the dllHell database.
- DlgDtxt2DB. exe: This program gives you a graphical user interface (GUI) version of the Dtxt2DB exe utility.
The DLL Help database helps you locate specific versions of DLLs that are installed by Microsoft software products.
This section describes the issues and the requirements that you should consider when you develop your own DLLs.
There are two ways to link a DLL into an application so that you can call the exported DLL functions. The two methods of linking are load-time dynamic linking and run-time dynamic linking.
In load-time dynamic linking, an application makes explicit calls to exported DLL functions like local functions. To use load-time dynamic linking, provide a header (. h) file and an import library (. lib) file when you compile and link the application. After you do this, the linker will give the system the details it needs to load the DLL and find the exported DLL function locations during load time.
When an application wants to load a DLL at run time, it calls either the LoadLibrary function or the LoadLibraryEx function. Once the DLL has been loaded, you can use the GetProcAddress function to find the address of the exported DLL function you want to call. When you use run-time dynamic linking, you do not need an import library file.
The following list shows the situations in which load-time dynamic linking and run-time dynamic linking should be used:
- Startup performance: If you care about how quickly your app starts up, you should use run-time dynamic linking.
- Because they are easy to use, exported DLL functions work the same way as local functions in load-time dynamic linking. This makes it easy for you to call these functions.
- With run-time dynamic linking, an application can choose to load different modules based on its needs. It is important when you develop multiple-language versions.
What Are DLLs?
FAQ
What do dynamic link library dll files do?
What is the purpose of a dynamic linked library?
How can a dynamic link library be loaded during run time?
What is Dynamic Link Library container for?
What is dynamic link library (DLL)?
Dynamic Link Library (DLL) is a library that consists of code that needs to be hidden. The code is encapsulated inside this library. An application can consist of many DLLs which can be shared with the other programs and applications. You can download a PDF version of Dot Net Interview Questions. Your requested download is ready!
What is dynamic link library?
Dynamic Link library or DLL in short are the extensions of applications. We often have common code between many applications and thus we put this common section of the code in an extension executable called DLL. The term DLL is very popular in Windows. However they are also present in Linux and Unix and known as Shared Libraries.
What is the difference between Exe and dynamic link library?
EXE is an executable file that runs the application for which it is designed. An EXE is produced when we build an application. Therefore the assemblies are loaded directly when we run an EXE. However, an EXE cannot be shared with the other applications. Dynamic Link Library (DLL) is a library that consists of code that needs to be hidden.
What are the two methods of linking a DLL?
When you load a DLL in an application, two methods of linking let you call the exported DLL functions. The two methods of linking are load-time dynamic linking and run-time dynamic linking. In load-time dynamic linking, an application makes explicit calls to exported DLL functions like local functions.
